There is a great need for high performance heat exchangers which increase the efficiency of utilization of waste heat in low temperature heat recovery applications as well as in low temperature power and refrigeration cycles. In addition to the efficiency gains by modification of heat exchangers by using enhanced surfaces, such as attached/integral fins, porous coatings, reentrant cavities and internal groovings, electrohydrodynamic (EHD) techniques have been developed which operates by applying a high voltage electrostatic potential field across a heat transfer fluid, which in one case may be a refrigerant or refrigerant mixture.
Active heat transfer enhancement techniques utilizing electric fields have been the subject of active research in recent years. Research has successfully demonstrated that application of electric fields to heat transfer surfaces and heat transfer media dramatically enhances the heat exchange processes and increases the efficacy of heat transfer systems. The phenomena of heat transfer enhancement has been studied and explained from the viewpoint of electrohydrodynamics (EHD), which relates to the interactions among electric fields, heat transfer media and its flow fields, and temperature fields. The applied electric field serves to increase mixing within the bulk flow and in particular within the fluid boundary layer, resulting in substantial increase of the heat and mass transport coefficients at the heat transfer surface. The resulting enhanced heat/mass transfer coefficients are often an order of magnitude higher than those achievable by conventional enhancement techniques.
For example, in Akira Yabe, et al., “Active Heat Transfer Enhancement by Utilizing Electric Fields”, Annual Review of Heat Transfer, Begell House, Vol. 7, 1996, pp 193–244, the active local EHD generation of turbulence was fundamentally described. In this Publication, the EHD effects on condensation and boiling were chosen as the representative interfacial EHD phenomena and these mechanisms were explained both theoretically and experimentally. In Ohadi, et al., “Electrode Design, Fabrication, and Materials Science for EHD-Enhanced Heat and Mass Transport”, Annual Review of Heat Transfer, Begell House, Vol. 11, pp. 563–623, the various design of electrode for enhancement of various single phase and phase change heat and mass transfer are described.
The electrohydrodynamic EHD technique is a promising technique which has proven potential for liquid pumping in the absence of any moving mechanical parts. Utilizing the effect of a phenomenon known as liquid extraction, the EHD technique can also effectively sustain the liquid pumping on a micro level, for example, in micro-electric mechanical systems (MEMS). Additionally, EHD has been demonstrated to show significantly enhanced heat transfer. Thus, the EHD technique can be used to enhance liquid pumping capabilities and simultaneously improve the cooling rates due to substantially higher heat transfer coefficients. The combination of these two capabilities make the use of the EHD a viable emerging technology for high performance heat exchanger device, including electronic cooling applications. Due to its lack of moving parts, this technology is highly reliable and is substantially maintenance free. Low cost and low power consumption are additional benefits of the EHD technique. Its applicability to heat transfer enhancement of industrially significant substances such as air, refrigerants, and certain aviation fuels has been previously demonstrated.
Unlike other techniques, the EHD technique provides on-line/on-demand, variable capacity control for heat exchange devices. Commercialization of this technique for selected applications in the near future is highly promising.
Moreover, EHD has significant potential in the control and enhancement of mass transfer, melting/solidification, and crystal growth. A very important area of EHD application is frost minimization. The EHD effects on frosting and defrosting phenomena were explained in Yabe, et al., and Ohadi, et al., supra. It is known that frost formation in a beat exchanger seriously affects the performance of the thermal system. This is an especially important problem in super market and food preservation refrigeration, food transportation refrigeration, as well as when applying heat pump systems to colder regions. The existing pre-EHD defrosting techniques use additionally imposed resistive heating to melt the frost on the heat transfer surface. To shorten the time required for melting and to minimize the additional energy required, a number of techniques have been researched and optimized from the engineering point of view. From the microscopic viewpoint, an important key issue is the removal of the dendritic crystals of ice from the heat transfer surface. Mostly Coulomb forces generated by an imposing electric field on the ice dipoles and accumulated charges at the interface between the heat transfer surface and the working media has the potential to remove the dendritic ice crystals. Frost formation has been shown to decrease more than 30% by application of the electrical field. The viability of completely eliminating frost formation by using the EHD technique is currently being studied.
Most of the EHD effects employ the electrical body force, which consists of three components: Coulomb forces, electrostrictive and electrophoretic forces. The Coulomb force is mostly related to electrical charges and dipoles. Other forces are related to the non-uniformity of the electrical fields or variation in fluid properties due to impurities, temperature gradients or other effects. Different parts of the body force contribute differently to the enhancement of different heat and mass transfer processes. This contribution depends on the working fluid's properties and the geometry of electrodes. For example, single phase gas heat transfer augmentation may be achieved mostly by dipole and ion movement as a result of the applied electrical field current. With two phase flow, the major contribution to the process can be attributed to the fluid properties or electrical field non-uniformity where the contribution of electrical current is small.
The EHD technique owes its particular application for enhancing heat transfer processes to different heat transfer mechanisms. For example, as described in U.S. Pat. No. 4,401,148, the efficiency of condensation heat transfer was notably augmented by opposing at least one electrode across a prescribed space to the heat transfer surface and applying a high electric potential capable of producing a non-uniform electric field. This enabled the condensate liquid formed on the heat transfer surface to be attracted by virtue of the electric field to the electrode, formed into a liquid column and removed from the heat transfer surface.
Further, in U.S. Pat. No. 4,396,055, a pumped heat pipe is electrohydrodynamically shown to be improved by means of application of a traveling potential wave thereto, thus inducing a traveling wave of electrical charge in the liquid phase of a dielectric working fluid which provides an electrical attraction which pumps the working fluid from a condensing section to an evaporating section of the electrohydrodynamic inductively pumped heat pipe.
U.S. Pat. No. 4,056,142 describes a heat exchange arrangement for use with chemically aggressive fluids wherein applying the principles of EHD to such a heat exchange arrangement decreases the effect of chemically aggressive fluids.
U.S. Pat. No. 4,207,942 discloses a plate heat exchanger which uses an anodic protection for the purposes of corrosion protection of plate heat exchangers formed particularly of stainless steel or titanium. In the arrangement of the plate heat exchanger comprising a pack of gasketed metal plates having aligned apertures to form supply and discharge ports for the heat exchange media, there is provided at least one electrode mounted in a manner to be insulated from the metal of the plates and extending along one of the ports formed by the aligned apertures.
EHD principles have also found utility in augmentation of boiling heat transfer, as described in U.S. Pat. No. 4,471,833 wherein the electric field is applied to a heat exchange medium so that the relaxation time of an electric charge of a heat exchange medium is made equal to or smaller than the characteristic time with respect to motion of bubbles generated by the heat transfer surface in the heat exchange medium which optimizes the maximum boiling heat flux.
U.S. Pat. No. 4,072,780 is directed to a method and apparatus for augmentation of convection heat transfer in liquid. In this arrangement, the electrodes are separated by spaces through which a liquid flows into/out of the heat transfer apparatus. A high voltage direct current is applied to the electrodes to produce turbulent components in the flow of the liquid to augment heat transfer between the liquid and the heat transfer surface.
One of the serious problems of the heat exchangers used in commercial and industrial refrigeration systems is directed to frost forming on the heat exchanger surface whenever the surface temperature is below the freezing point of water. The formation of frost in heat exchangers has been and continues to be a serious and vexing problem. The effects of the problems created by frost formation are typified by the general trends observed during the course of test runs conducted at a fixed mass flow rate of air, where it was found that there was a decrease in heat transfer rate with frost formation. The frost formation negatively affects the heat transfer of the heat transfer system due to frost-induced additional heat transfer resistance at the surface and the reduction in free flow area between the working media and heat exchange surface.
Among other techniques to reduce frost formation in the refrigeration type heat exchangers, the utilization of electrostatics is most promising to control the direction of droplets comprising a liquid fluid stream, which is the cause of the frost formation such as dendritic, crystalline, or ice-like structures on the heat exchange surfaces.
In U.S. Pat. No. 3,681,896, the control of frost formation in heat exchangers is carried out by applying an electrostatic charge to the air stream and to water introduced into the stream. The charged water droplets induce coalescence of the water vapor in the air. An electrical potential is applied to repel the charged fluid and in the next region, the surface is at a potential to attract the charged vapor thereby permitting the air stream to pass to the heat exchanger vapor free, thus substantially preventing frost formation on the heat exchange surface.
In the apparatus for electrohydrodynamic augmentation of heat transfer, described in U.S. Pat. No. 5,769,155, a heat exchange surface is provided with fins forming a space therebetween at which a working fluid flows with a flow velocity sufficient to form a thermal boundary layer in the vicinity of the heat transfer surface. The electrode wire is located in the space formed between the fins of the heat exchange surface. The electrode is coupled to a controllable source of high voltage, while the heat exchange surface may be electrically conductive and act as a ground. Insulators formed as spherical beads, each having a central opening sized for receiving the electrode wire therethrough, are spaced apart longitudinally along the electrode wire. The insulators are spaced along the electrode wire at a predetermined distance and have a diameter sufficient to distance the electrode wire from the heat exchange surface and the surface of the fins referred to as a minimum stand off distance.
Under normal operation conditions, the working fluid in contact with the walls of the fins forms a disruptible layer adjacent to the heat exchange surface. The voltage source produces an electrical field around the electrode wire. The electrical field is highly non-uniform due to the angled shape of the spacing between the fins. The non-uniform field thereby disrupts the disruptible layer adjacent to the heat exchange surface, thereby enhancing the heat exchange between the heat exchange surface and the working fluid.
Despite using the electrohydrodynamic enhancement of the heat transfer, the apparatus described in '155 patent, as well as the other EHD enhanced systems suffer from several drawbacks. Notably, the electrode wire is only partially insulated from the working fluid and has a conductive surface submerged in the working fluid. Such an arrangement may cause short circuiting between the electrode and the heat exchange surface if the heat transfer media is too conductive or if there are conductive impurities in the media which may be deposited between the electrode wire and the heat exchange surface (which is grounded for EHD purposes).
The conductive impurities may include metal particles from the welding which is often the case in industrial units. Such metal particles develop between the electrode wire and the heat exchange surface and can short-circuit the system. Another drawback is the possible current leakage, as well as ion recombination on the electrode wire, resulting in higher power consumptions for the EHD effect. Additionally, electrochemical corrosion of electrodes may lead to premature breakdown of the electrodes and working media.
Disadvantageously to all above described EHD applications, these techniques use bare electrodes adjacent to the heat exchanging surface. The bare electrodes are covered with condensed water, which is a good conductor, and as a result, these systems suffer high leakage currents, surface discharge and sparking, that substantially increases high voltage power consumption which may be up to 2 or 3 orders of magnitudes.
As a result, the achieved electrical field strength is low and the influence of the electrical field for augmentation of heat transfer processes is insufficient. For effective heat transfer in EHD-enhanced heat transfer systems, it is important to provide a high voltage electric field between the electrodes of the system across the working media, as presented in J. T. Bartlett, et al. “The Growth of Ice Crystal in an Electric Field”, Zeltschrift fur Angehandte Mathematik und Physik, Vol. 14, Pages 599–610, 1963, and Akira Yabe, et al. “Active Heat Transfer Enhancement by Utilizing Electric Fields”, Annual Review of Heat Transfer, Vol. 7, 1996, Pages 193–244. In both Publications, it has been shown that the effect of EHD technique is highly dependent on supporting a sufficient level of the electric field which in most cases is not possible with the bare electrode of the previously described EHD system. It would be therefore highly desirable to have an EHD heat and mass transfer technique free of the disadvantages of the prior art.